Threatening (to discover) quantum gravity with a big metal bar

Vibrations of AURIGA detector set limits on deviations from quantum mechanics.

The AURIGA gravitational wave detector consists of a 3-meter aluminum cylinder, cooled to a few thousandths of a degree above absolute zero. The bar is kept carefully isolated from other vibrations, to help measure tiny gravitational disturbances - or possibly quantum-gravitational effects.

Despite many decades of effort, quantum physics and gravitation remain separate: no consistent quantum theory of gravity exists. However, that doesn't mean we don't have hints about what such a theory would look like. In particular, many proposals that are wildly different in their details agree that there is a fundamental length scale—the Planck length—at which the force of gravity becomes as strong as the other forces. Close to the Planck scale, ordinary physics, including the famous Heisenberg uncertainty principle, breaks down. Exactly how this breakdown happens is presently (wait for it!) uncertain.

When theory is silent, experiment must step in. A new paper analyzed results from the AURIGA gravitational wave experiment to check for deviations from standard quantum mechanics in the vibrations of a massive metal bar at cryogenic temperatures. The AURIGA results showed no deviation from standard quantum physics, yielding an upper bound on the energy of quantum gravity modifications. The experimenters concluded that the theorists needed to get back to work so that the experimenters have a better idea of what to expect.

So far, experimenters and theorists haven't had much to talk about. This is in part because the Planck length is extremely tiny: 1.6×10-35 meters, compared with the 10-15 meter or so occupied by an atomic nucleus. The particle physics approach to probing short distances is to crank up the energy of collisions, but these energies are well beyond the capabilities of any collider—including the Large Hadron Collider (LHC).

As a result, researchers have turned to other methods to search for quantum-gravitational effects. One such technique involves observing extremely high energy astrophysical events, though these carry their own measurement problems. In the laboratory, the focus has been on collective behavior in very cold systems. (For one such proposal, see my earlier Ars Technica article.)

AURIGA is the Antenna Ultracriogenica Risonante per l'Indagine Gravitazionale Astronomica, or "ultra-cryogenic resonant antenna for gravitational-wave astronomy." The detector consists of an aluminum cylinder 3 meters in length and weighing approximately 2.3 tons. The whole apparatus is cooled to a few thousandths of a degree above absolute zero to minimize the effect of thermal vibrations of its atoms. When a gravitational wave from a cataclysmic astronomical event reaches the detector, the aluminum bar should vibrate at its resonant frequency, like a wine glass when a certain musical note is played.

(To date, no gravitational waves have been detected directly, either by AURIGA or the larger laser-based detectors such as LIGO.)

Although this is not its primary purpose, AURIGA's ultra-cryogenic operation temperature made it a good candidate for searching for quantum-gravitational effects. According to quantum physics, mechanical systems will vibrate even at absolute zero, exhibiting something known as zero-point energy. If the Heisenberg uncertainty principle is modified by quantum gravity, then the zero-point energy would also be changed—and it's possible we could measure its increase.

The present analysis found an upper limit for quantum-gravitational effects in the AURIGA data by determining the vibrational energy of the aluminum bar. This is accomplished by coupling the bar to sensitive electrical readout instruments, which were designed for the detection of gravitational waves.

The limit the researchers found was not very stringent: there's still a lot of room for deviation from quantum mechanics at the resolution provided by this experiment. The lack of stringency in these limits leaves open the question of whether AURIGA is sensitive enough to measure quantum gravitational effects. However, these results are a proof of principle, since this stands as the first use of a very macroscopic body to probe physics at the smallest physical scales.

As the researchers emphasized, theory needs to catch up now. While some quantum gravity schemes make specific predictions for deviations from the Heisenberg uncertainty principle, they are often based on idealized systems or single-particle measurements. Promising techniques, including the AURIGA experiment, harness collective phenomena: the behavior of many coupled atoms. Theorists need to catch up by taking into account many-particle effects and measurement processes before any experimental deviation from quantum mechanics could be interpreted.

Promoted Comments

Searching for gravitational waves always makes me think of the Michelson–Morley experiment. They went through enormous effort to removal outside effects, but could never seem to measure the ether. Years later, it was shown that there was no ether. Is there any chance that we will come up with a new theory that says these billions were spent looking for something that doesn't exists? Not to say that experiments with negative results are not worth performing, I'm just curious if my great grandchildren will think of these experiments instead of Michelson-Morley.

You never know until you perform the experiment. Michelson-Morley experiment "failure" started new research that gave us special relativity. It was extremely important experiment.

Is there any chance that we will come up with a new theory that says these billions were spent looking for something that doesn't exists?

Of course- but people who claim that MM was a failure are missing the point. Michelson–Morley was a total success insofar as it unequivocally demonstrated to theorists what *not* to consider in resolving the problems with galilean invariance in the maxwell field equations. Eliminating the impossible is just as helpful as illuminating the possible. Remember, the next step after Michelson-Morley was the Lorentz transformations and special relativity itself.

So, getting back to quantum gravitation, the situation is similar. Einstein's field equations for general relativity are known to not work alongside quantum electrodynamics. Either a positive or a negative result from experiments such as this will point the way forward for theorists.

Searching for gravitational waves always makes me think of the Michelson–Morley experiment. They went through enormous effort to removal outside effects, but could never seem to measure the ether. Years later, it was shown that there was no ether. Is there any chance that we will come up with a new theory that says these billions were spent looking for something that doesn't exists? Not to say that experiments with negative results are not worth performing, I'm just curious if my great grandchildren will think of these experiments instead of Michelson-Morley.

Searching for gravitational waves always makes me think of the Michelson–Morley experiment. They went through enormous effort to removal outside effects, but could never seem to measure the ether. Years later, it was shown that there was no ether. Is there any chance that we will come up with a new theory that says these billions were spent looking for something that doesn't exists? Not to say that experiments with negative results are not worth performing, I'm just curious if my great grandchildren will think of these experiments instead of Michelson-Morley.

You never know until you perform the experiment. Michelson-Morley experiment "failure" started new research that gave us special relativity. It was extremely important experiment.

Searching for gravitational waves always makes me think of the Michelson–Morley experiment. They went through enormous effort to removal outside effects, but could never seem to measure the ether. Years later, it was shown that there was no ether. Is there any chance that we will come up with a new theory that says these billions were spent looking for something that doesn't exists? Not to say that experiments with negative results are not worth performing, I'm just curious if my great grandchildren will think of these experiments instead of Michelson-Morley.

We've already measured them indirectly. The in spiral of several binary pulsars matches general relativity's predictions of energy loss due to gravitational radiation to within the limits of experimental data (a few percent).

For direct measurement, ask us again in a decade. Advanced Ligo is on schedule to begin collecting data in 2014. Assuming their analysis timeline remains relatively unchanged, they should have initial results published about 3 years later (1 year for data collection and preprocessing, 1 year to do the main computational analysis through Einstein@Home, and 1 year for post-processing and analysis). The gravitational radiation generated by mountains (all of several meters high) on several nearby pulsars are expected to be well over Advanced Ligo's detection threshold.

Is there any chance that we will come up with a new theory that says these billions were spent looking for something that doesn't exists?

Of course- but people who claim that MM was a failure are missing the point. Michelson–Morley was a total success insofar as it unequivocally demonstrated to theorists what *not* to consider in resolving the problems with galilean invariance in the maxwell field equations. Eliminating the impossible is just as helpful as illuminating the possible. Remember, the next step after Michelson-Morley was the Lorentz transformations and special relativity itself.

So, getting back to quantum gravitation, the situation is similar. Einstein's field equations for general relativity are known to not work alongside quantum electrodynamics. Either a positive or a negative result from experiments such as this will point the way forward for theorists.

The whole apparatus is cooled to a few millionths of a degree above absolute zero to minimize the effect of thermal vibrations of its atoms.

Was wondering how they did that. Turns out they don't. It's cooled by a dilution refrigerator to ~ 100mK.

I was just wondering the same thing. Microkelvin cooling of macroscopic is really really hard, especially "a few" microkelvin. 100 mK in a dilution fridge, that's pretty close to off-the-shelf (not quite, since stock commercial dil fridges don't tend to be very large, but for a sufficient amount of money they can be scaled up).

Edit: Reading through the paper and looking at a couple of the references, I see that we're both wrong. The thermal temperature of the test mass is 4.2 K, cooled via a standard liquid helium bath. Certain vibrational modes of the bar are damped down by various clever techniques to an effective temperature of under 1 mK, and that's what they're using to get at approaching the detection limits as a sensor for gravity waves.

Searching for gravitational waves always makes me think of the Michelson–Morley experiment. They went through enormous effort to removal outside effects, but could never seem to measure the ether. Years later, it was shown that there was no ether. Is there any chance that we will come up with a new theory that says these billions were spent looking for something that doesn't exists? Not to say that experiments with negative results are not worth performing, I'm just curious if my great grandchildren will think of these experiments instead of Michelson-Morley.

You never know until you perform the experiment. Michelson-Morley experiment "failure" started new research that gave us special relativity. It was extremely important experiment.

Or in other words, all experimental outcomes are useful data. That something fails can be more important than having it work.

"It’s often said that it is difficult to reconcile quantum mechanics (quantum field theory) and general relativity. That is wrong. We have what is, for many purposes, a perfectly good effective field theory description of quantum gravity. It is governed by a Lagrangian ...

Nonetheless, at low energies, i.e., for ε ≡ E^2/Mpl^2 << 1, we have a controllable expansion in powers of ε. ...

In other words, as an effective field theory, gravity is no worse, nor better, than any other of the effective field theories we know and love.

The trouble is that all hell breaks loose for ε ~ 1."

I take it [having no general relativity, no quantum field theory studies] this means that outside black holes, where the membrane paradigm kicks in a Planck length away from the event horizon, semiclassical physics is "safe". [ http://en.wikipedia.org/wiki/Membrane_paradigm ]

Since in cosmology reheating after inflation gives spacetime particles with temperatures far away from the Planck energy (I've seen "Starts With A Bang" claim some 5 oom away IIRC), the potential energy density before would be the same I take it. So inflation is a semiclassical and "safe" spacetime and yielding everything outside black holes a spacetime, isn't it?

When we proceed to small dimensions of spacetime and its quantization, I note that quantum entanglement has inconsistencies with spacetime. Entangled systems makes spacetime separated volumes behave microscale local, under spacetime causality, in a cosmologically local and temporary fashion (so no problem with cosmological spacetime). Or so Bell test experiments seems to say.

And in fact I think Penrose's twistor theory simply maps volumes to join such volumes, so being a more fundamental space than spacetime. [But as I noted, I don't really know the ins and outs of any of this.]

I don't think this is semantics.

String theory too seems to describe a consistent quantum gravity theory, where it is spacetime that is emergent above Planck scales but gravitons still exist at all scales. And the observation of "extremely high energy astrophysical events" seems to concur that the underlying manifold is smooth and without quantum fluctuations at or below Planck scale. (Roughly 3 sigma experiments, IIRC. 'Nuff to test a smoothness theory.)

Isn't it simply that spacetime as an object is less well understood than gravity as an interaction? Eg black holes is a red herring, spacetime is non-intuitive everywhere.

And the Planck scale is, again simply, the scale where particle fields have a cut off for particle masses. I.e. their observable ability to be disturbed in the manner of particles, as "a nice, regular ripple" to quote Strassler. [ http://profmattstrassler.com/articles-a ... -are-they/ ] But the existence of particles is not tied to gravity as such, it seems to me.*

* Which makes it easier IMHO to understand the non-particle time of inflation before reheating, however it ties to Planck energy density scales. The GR part of standard cosmology is still there, I take it.

Searching for gravitational waves always makes me think of the Michelson–Morley experiment. They went through enormous effort to removal outside effects, but could never seem to measure the ether. Years later, it was shown that there was no ether. Is there any chance that we will come up with a new theory that says these billions were spent looking for something that doesn't exists? Not to say that experiments with negative results are not worth performing, I'm just curious if my great grandchildren will think of these experiments instead of Michelson-Morley.

While that's always possible, the Michelson—Morley experiment was a walk in the park compared to this. Another difference is that we really do know enough to be able to rule out a lot of things, whereas they knew very little.

It's not likely that Quantum Gravity doesn't exist at all. It's that whatever it exactly is, is at such low levels that it's going to be almost impossible to detect with present technology. There were a number of things that Relativity threw out that were just thought experiments until technology grew up to the task of verifying them. But it did!

I'm not sure why Aluminum was picked over a denser metal; but at the extreme sensitives for vibration they're targeting the recoil of uranium atoms as they decay (first into thorium, with several other decays happening afterwards in short succession) could represent an additional noise source degrading the data.

Eliminating the impossible is just as helpful as illuminating the possible.

Quite. But I think it is the different characters of the results that are confusing.

Rejecting the impossible under such-and-such constraints has more theoretic leverage. The possible can still have several viable candidates living before and after being sorted out. (For an example of the latter gravitation serves - classical gravity, linearized GR, GR. It is cheaper to use CG to throw spacecrafts around, while GPS needs linearized GR.)*

Accepting the possible (even when it isn't entirely "correct") has more practical leverage. Apps, apps, apps.

* I find it fascinating, and of course useful, that the elimination process converges despite our very finite resources in an infinite universe. The standard Higgs, if that is what it is, is the final nail in building the understanding of the laws of everyday physics, as Sean Carroll likes to put it. (He has a youtube out on that already. Apps, apps, apps.)

Not only does science work, it works the best we can hope for. Now someone needs to turn its instruments on itself and figure out how and why it works. 'Tis a mystery, not rejection from testing which is well understood, but the convergence of serial rejection.

In the generic sense described here which roughly is gravity = spacetime, yes, its existence should be unlikely IMHO.

- If spacetime was quantized it would show up as spacetime fluctuations in those high-energy cosmological distance experiments, or in inflation besides the inflation field fluctuations observed in the CMB. Instead spacetime and whatever exists under Planck scales looks perfectly smooth.

a) Quantization works on similar particle fields (because in some instances gravity and particle fields look analogous, I think).

b) It would be helpful for those who wishes for a predictive Theory Of Everything, to nail down the origin of the universe (in some wishes) and the parameters of the Standard Model.

But:

If one wish to be cruel, a) is Einstein's last years all over again. And they bat 0 on b), while inflation looks quite fruitful on the predictivity area. I don't think you can realistically dodge the prediction of the cosmological constant. And that is for starters.

But what I deem unlikely is not what turns these fields around. Any which way, the science moves forward. It is interesting how an outsider's view can so totally not understand the inner workings of the field.

There seems to be some misinformation and confusion here.Using the rseonance of gravitational waves interaction with aluminum cylinders was tried as long ago as 50 years by Joseph Webber of the University of Maryland. Although Webber claimed some detections, no other experimenter was able to confirm his results. It was my impression aluminum cylinder resonance detectors have been largely abandoned.That gravitational radiation exists has been indirectly proven from decaying orbits of Hulse-Taylor binaries. The 1993 Nobel Prize was awarded for their discovery and analysis. The analysis is very similar to electro-magnetic theory. Just as an accelerated charge radiates energy and results in the orbital decay of electrons in atoms (to the "ground" state) accelerated masses also radiate energy and the orbits decay.The detection of gravitons is not possible however because of their low cross-section. I have read discussions that a graviton can easily pass through 50 light years of lead and that a detector capable of detecting one graviton per year would be about the size of the solar system.I know of three "possible" detectors of graviataional waves: LIGO/Advanced LIGO, LISA and TOBA. All work by measuring the strain of space-time as a gravitational wave passes the detector. The strain is the change in length per unit length.Because the strain is extremely small (e-20) the detectors need to be very sensitive LIGO or TOBA or very large LISA or both. Advanced LIGO should begin producing results in several years. LISA and TOBA are not likely to be online until 2030.

Is there any chance that we will come up with a new theory that says these billions were spent looking for something that doesn't exists?

One nitpick: I think "billions" is an overestimate. The largest gravity wave experiment, LIGO, has a cost of $365 million for a pair of multi-kilometer vacuum chambers operating in sync 3000 km apart. Pretty sure a 3 meter aluminum cylinder in a refrigerator costs a lot less than that.

One thing I find confusing is the theories that predict the existance of gravitons.

In quantum mechanics, the electromagnetic, strong and weak nuclear forces are controlled by exchanging force carrying particles that have zero mass and travel at the speed of light.

In general relativity, gravity is caused by the curvature of space-time resulting from the presence of energy-momentum. To put it another way: space-time curvature results from mass carrying particles.

These are consistent with the standard model that groups force carrying particles in a different group to mass carrying particles.

This is not to say that the theories are complete. Indeed, black holes and the early universe are two places where the two interact and break down. Black hole physics is interesting as there are phenomena like Hawking radiation, and understanding the structure of the black hole inside the event horizon (which we cannot observe directly) that hint at where unification of quantum mechanics and general relativity lies.

Gravity waves are the ripples in space time, like the ripples of disturbances on water.

In general relativity, gravity is caused by the curvature of space-time resulting from the presence of energy-momentum. To put it another way: space-time curvature results from mass carrying particles.

Huh? Gravity causes the curvature of space, not the other way around. Gravity causes acceleration, which via general relativity, causes the curvature of space. The acceleration must exist or space would not be curved.

In quantum mechanics, the electromagnetic, strong and weak nuclear forces are controlled by exchanging force carrying particles that have zero mass and travel at the speed of light.

This is within spitting distance of true for the electromagnetic and strong interactions. A counterexample is Compton scattering, which occurs via fermion exchange between a photon and charged fermion.

It's completely, 100% false for the weak interaction. The Z and W bosons are the 3rd and 4th heaviest particles known, respectively, and the Higgs is the 2nd most massive.

Quote:

In general relativity, gravity is caused by the curvature of space-time resulting from the presence of energy-momentum. To put it another way: space-time curvature results from mass carrying particles.

Massless particles do contribute as gravitational sources in general relativity, for which the source term is not mass but the stress energy tensor. Photons contained in the sun add an additional component to its gravitational force. Mass-carrying particles dominate the source of gravitational fields because rest energies are typically much larger than the energies carried by massless particles, but consider this: where does the mass of a proton or neutron, the vast majority of the mass of the sun, come from? Mostly from the kinetic energies of the quarks and gluons composing them, NOT from the mass of their constituent particles. Gravity has nothing to do with mass, except insofar as mass is one form of energy.

Quote:

These are consistent with the standard model that groups force carrying particles in a different group to mass carrying particles.

The Standard Model does not do this. At all. It does distinguish between bosons and fermions in a way, but bosons are both massive (W,Z, Higgs) and massless (photon, gluon), and fermions are all massive, with the possible but unlikely exception of at most 1 of the neutrinos.

Physicists suspect the existence of the graviton because, if you write down the equations for a massless spin-2 particle, you get general relativity.

In general relativity, gravity is caused by the curvature of space-time resulting from the presence of energy-momentum. To put it another way: space-time curvature results from mass carrying particles.

Massless particles do contribute as gravitational sources in general relativity, for which the source term is not mass but the stress energy tesnor. Photons contained in the sun add an additional component to its gravitational force. Mass-carrying particles dominate the source of gravitational fields because rest energies are typically much larger than the energies carried by massless particles, but consider this: where does the mass of a proton or neutron, the vast majority of the mass of the sun, come from? Mostly from the kinetic energies of the quarks and gluons composing them, NOT from the mass of their constituent particles. Gravity has nothing to do with mass, except insofar as mass is one form of energy.

Mr. Law

It's safe to say that a theoretical GRB that has a energy equivalent of the earth using E = m.c² where m is the earth mass would generate distortions equal to 1G?

In general relativity, gravity is caused by the curvature of space-time resulting from the presence of energy-momentum. To put it another way: space-time curvature results from mass carrying particles.

Huh? Gravity causes the curvature of space, not the other way around. Gravity causes acceleration, which via general relativity, causes the curvature of space. The acceleration must exist or space would not be curved.

The equation maps space-time curvature (the tensor R on the left-hand side) as a function of energy-momentum (the tensor T on the right-hand side). Given that `E = mc^2`, energy and mass are equivalent.

Therefore, an object that carries mass (e.g. the Earth) curves space-time. Another object's motion will be deviated by this curvature. For example, the moon is travelling in a straight line, but ends up in a circular orbit due to the curviture of space-time. It is also why Mercury has the orbit it does.

Oh, and momentum is mass times velocity and velocity is increased by acceleration, so the faster an object is moving, the higher its momentum and thus the greater its gravitational effect.

One thing I find confusing is the theories that predict the existance of gravitons.

In quantum mechanics, the electromagnetic, strong and weak nuclear forces are controlled by exchanging force carrying particles that have zero mass and travel at the speed of light.

In general relativity, gravity is caused by the curvature of space-time resulting from the presence of energy-momentum. To put it another way: space-time curvature results from mass carrying particles.

These are consistent with the standard model that groups force carrying particles in a different group to mass carrying particles.

This is not to say that the theories are complete. Indeed, black holes and the early universe are two places where the two interact and break down. Black hole physics is interesting as there are phenomena like Hawking radiation, and understanding the structure of the black hole inside the event horizon (which we cannot observe directly) that hint at where unification of quantum mechanics and general relativity lies.

Gravity waves are the ripples in space time, like the ripples of disturbances on water.

That's how my (layman) brain sees it: gravity waves are ripples in a medium, much like sound. I wouldn't expect there to be a messenger particle. But I'm sure a fourth-year physics student could school me on where I've gone wrong.

In quantum mechanics, the electromagnetic, strong and weak nuclear forces are controlled by exchanging force carrying particles that have zero mass and travel at the speed of light.

This is within spitting distance of true for the electromagnetic and strong interactions. A counterexample is Compton scattering, which occurs via fermion exchange between a photon and charged fermion.

It's completely, 100% false for the weak interaction. The Z and W bosons are the 3rd and 4th heaviest particles known, respectively, and the Higgs is the 2nd most massive.

Quote:

In general relativity, gravity is caused by the curvature of space-time resulting from the presence of energy-momentum. To put it another way: space-time curvature results from mass carrying particles.

Massless particles do contribute as gravitational sources in general relativity, for which the source term is not mass but the stress energy tensor. Photons contained in the sun add an additional component to its gravitational force. Mass-carrying particles dominate the source of gravitational fields because rest energies are typically much larger than the energies carried by massless particles, but consider this: where does the mass of a proton or neutron, the vast majority of the mass of the sun, come from? Mostly from the kinetic energies of the quarks and gluons composing them, NOT from the mass of their constituent particles. Gravity has nothing to do with mass, except insofar as mass is one form of energy.

Quote:

These are consistent with the standard model that groups force carrying particles in a different group to mass carrying particles.

The Standard Model does not do this. At all. It does distinguish between bosons and fermions in a way, but bosons are both massive (W,Z, Higgs) and massless (photon, gluon), and fermions are all massive, with the possible but unlikely exception of at most 1 of the neutrinos.

Physicists suspect the existence of the graviton because, if you write down the equations for a massless spin-2 particle, you get general relativity.

It's safe to say that a theoretical GRB that has a energy equivalent of the earth using E = m.c² where m is the earth mass would generate distortions equal to 1G?

Is that it?

If you contained the gamma ray burst photons in a nearly massless Earth shaped mirror system (thought experiment), yes, the field would look the same as Earth's from outside the mirror system, as it would be fundamentally no different from the proton's relationship with gluons. If you allowed them to have net motion, their contributions would show up in different places in the stress energy tensor than would mass, and so you'd get a different contribution.

You're welcome! I should add: why then is not the graviton a consistent theory of quantum gravity? It's because spin-2 theories are non-renormalizable. That is to say, at low energy they are perfectly fine, and you get general relativity. At high energy, they need something else to step in, similar to how the Higgs intervenes to save the weak interactions, or else you start getting infinity this and infinity that. In the same way, if you try to write down the equations for a W boson without also including a Higgs boson or something similar, the W-W scattering probability would eventually become greater than 1. While we have found something that works for the weak interaction, and even discovered it experimentally, we have no fricking clue what works for gravity. Well, string theory does work, but that's an aside....

Some confusion seems expressed in the comments between gravity waves and Quantum Gravity. Yes waves have been confirm as entirely consistent with observed orbital decay between close orbiting pulsars. But this requires only GR. The idea of Quantum Gravity is there is some minimum quantized size within spacetime because everything else appears to be quantized at very small scales and this minimum size gets rid of messy singularities that occur in GR for black holes. So Planck's constant was imposed as an assumed spacetime limit (a key supposition for String Theory). The use of Planck's Constant is really quite arbitrary since no theory derives this specific length as a reqirement. But fine test the assumption and a very cleaver test they came up with here. Nice for us to think spacetime is quantized at Planck's length avoiding to us messy singularities. But these ideas are just that until and if proven. Outside of inconvenient singularities Quantum and GR are only incompatible because of causality violation from entanglement and nonlocality due to Heisenberg. GR and Quantum don't necessarily have to meet, although we really seem to want them to. The latest results from high energy physics show no evidence that discrete spacetime exists within current observable limits. Nature is what it is. It will be interesting to see what it really is. There will certainly be surprises.

It's safe to say that a theoretical GRB that has a energy equivalent of the earth using E = m.c² where m is the earth mass would generate distortions equal to 1G?

Is that it?

If you contained the gamma ray burst photons in a nearly massless Earth shaped mirror system (thought experiment), yes, the field would look the same as Earth's from outside the mirror system, as it would be fundamentally no different from the proton's relationship with gluons. If you allowed them to have net motion, their contributions would show up in different places in the stress energy tensor than would mass, and so you'd get a different contribution.

So if the GRB are in a "beam shape" the gravity/spacetime curvature would be divided along the beam?

1) Torbjorn: "The trouble is that all hell breaks loose for ε ~ 1". This line that you quoted fundamentally explains why the author wrote "no consistent quantum theory of gravity exists." The fact that General Relativity simplifies usefully under most conditions we think of as "quantum realm" is not sufficient for it to be considered a complete quantum theory of gravity. There are various problems with quantum gravity -- what happens in a black hole? how do you add gravitons to the standard model without breaking it?

2) It's possible to write many pages about the confusions associated with the terms mass, gravity, energy and E=mc^2. Add the confusions associated with space-time, and we've got whole books. I think the best characterization of general relativity is "space-time tells matter how to move; matter tells space-time how to curve". The devil is in the details, though. The thing that often confuses people about both Special and General Relativity is that E=mc^2 does not mean that energy can be converted to mass and back. It means that energy is mass and mass is energy. A charged laptop battery actually has a greater mass, even though it has the same number of protons, neutrons and electrons as an uncharged laptop battery. (You wouldn't be able to measure the difference in mass, though. It's tiny.)

3) Gravity waves are actually ripples in space-time, the same way that photons can be thought of as ripples in E-M fields. Whether gravity's ripples are quantized into gravitons is an open question in physics. It would certainly be consistent with the other forces, but who says the universe has to be consistent? What we do know is that adding gravitons to the standard model or other QM models doesn't work. So we know that either those models are wrong or there are not gravitons.

4) If you constrained a bunch of photons with enough energy into a volume the size of Earth, you would only get an approximation of the same space-time curvature around the Earth. The densities of mass would occur at different locations. Also, the Earth's spin drags space-time around (like a spiral, sort of). But it is true that photons curve space-time just like any other particle, with the amount of curvature being related to energy content (rest mass + momentum).

So if the GRB are in a "beam shape" the gravity/spacetime curvature would be divided along the beam?

You'd get what's called a pp-wave metric. I haven't studied them, since the metrics for traditional sources--Schwarzschild, Kerr-Newman, etc--are much more important for practical purposes, so I shouldn't try to give a qualitative picture that might be wrong. I can say that, since the stress energy tensor would not be diagonal, you'd get some weird effects similar to frame dragging around a rotating black hole.

But i think pehaps your laptop battery analogy is wrong. A charged one would have more electrons that the uncharged one hence the difference in mass. no?

No. You're thinking about a capacitor which does store electrons. A battery stores energy in a chemical reaction. When it's powering something the chemical reaction produces an electric potential that causes electrons to flow from one terminal of the battery, through the device, and back into the other terminal. The net number of electrons remains (roughly) constant throughout the process.

Is there any chance that we will come up with a new theory that says these billions were spent looking for something that doesn't exists?

Of course- but people who claim that MM was a failure are missing the point. Michelson–Morley was a total success insofar as it unequivocally demonstrated to theorists what *not* to consider in resolving the problems with galilean invariance in the maxwell field equations. Eliminating the impossible is just as helpful as illuminating the possible. Remember, the next step after Michelson-Morley was the Lorentz transformations and special relativity itself.

So, getting back to quantum gravitation, the situation is similar. Einstein's field equations for general relativity are known to not work alongside quantum electrodynamics. Either a positive or a negative result from experiments such as this will point the way forward for theorists.

But it's quantum mechanics, what if the result is positive *and* negative and we end up with an undead zombie cat apocalypse?

I thought the Higgs field stuff took care of gravity (mass?). Obviously this is about detecting, but I thought the theory made a major step forward. Could someone briefly explain the relationship b/w this article and Higgs - like as if you were talking to a moron.

But i think pehaps your laptop battery analogy is wrong. A charged one would have more electrons that the uncharged one hence the difference in mass. no?

No. You're thinking about a capacitor which does store electrons. A battery stores energy in a chemical reaction. When it's powering something the chemical reaction produces an electric potential that causes electrons to flow from one terminal of the battery, through the device, and back into the other terminal. The net number of electrons remains (roughly) constant throughout the process.

Capacitors work similarly to batteries. To charge, you push electrons in one side, while pulling electrons out the other side. ("Push" and "pull" are probably terrible words to use there, but you get the idea.)

EDIT: And to address jaderanderson's comment: The energy stored in the chemical bonds contributes the extra mass to the charged battery. There's probably some Standard Model explanation for that extra mass: the energy of the photons carrying the electrostatic force between atoms, or some such.

I thought the Higgs field stuff took care of gravity (mass?). Obviously this is about detecting, but I thought the theory made a major step forward. Could someone briefly explain the relationship b/w this article and Higgs - like as if you were talking to a moron.

Thnx.

Mass and gravity are not the same. The Higgs field was mostly worked out decades ago, it's just been awaiting experimental confirmation. It doesn't do much of anything to advance a quantum theory of gravity.

I thought the Higgs field stuff took care of gravity (mass?). Obviously this is about detecting, but I thought the theory made a major step forward. Could someone briefly explain the relationship b/w this article and Higgs - like as if you were talking to a moron.

Thnx.

We already understood how masses interact in the form of gravity (with the exception of the problems associated with combining General Relativity and Quantum Mechanics, that is). But without the Higgs theory, the Standard Model doesn't really have an explanation for how/why some particles have rest mass and others don't.